The present invention relates to the design of wellbore strengthening additives and fluids based on the assessment of the properties of a plug comprising wellbore strengthening materials, including methods, apparatuses, and systems relating thereto. Generally, the properties of the plug may translate to the near wellbore strengthening effect of the wellbore strengthening materials of the plug.
Lost circulation is one of the larger contributors to non-productive drilling time. Lost circulation arises from drilling fluid leaking into the formation via undesired flow paths, e.g., permeable sections, natural fractures, and induced fractures. Lost circulation treatments may be used to remediate the wellbore by plugging the undesired flow paths before drilling can resume.
Drilling, most of the time, is performed with an overbalance pressure such that the wellbore pressure (equivalent circulating density) is maintained within the mud weight window, i.e., the area between the pore pressure (or collapse pressure) and the fracture pressure, see FIG. 1. That is, the pressure is maintained high enough to stop subterranean formation fluids from entering the wellbore and low enough to not create or unduly extend fractures surrounding the wellbore. The term “overbalance pressure,” as used herein, refers to the amount of pressure in the wellbore that exceeds the pore pressure. The term “pore pressure,” as used herein, refers to the pressure of fluids in the formation. Overbalance pressure is needed to prevent subterranean formation fluids from entering the wellbore. The term “fracture pressure,” as used herein, refers to the pressure threshold where pressures exerted in excess of the fracture pressure from the wellbore onto the formation will cause one or more fractures in the subterranean formation. Wider mud weight windows allow for drilling with a reduced risk of lost circulation.
In traditional subterranean formations, the mud weight window may be wide, FIG. 1. However, in formations having problematic zones, e.g., depleted zones, high-permeability zones, highly tectonic areas with high in-situ stresses, or pressurized shale zones below salt layers, which are often found in formations with a plurality of lithographies, the mud weight window may be narrower and more variable, FIG. 2. When the overbalance pressure exceeds the fracture pressure, a fracture is expected to be induced, and lost circulation may occur. One proactive method of reducing the risk of lost circulation is to strengthen or stabilize the wellbore through the use of wellbore strengthening materials. One method of wellbore strengthening involves inducing fractures while simultaneously plugging the fractures. This simultaneous fracture-plug method increases the compressive tangential stress in the near-wellbore region of the subterranean formation, which translates to an increase in the fracture initiation pressure or fracture reopening pressure, thereby widening the mud weight window, FIG. 3. The extent of wellbore strengthening, i.e., expansion of the mud weight window, could be a function of the properties of the plug in terms of its ability to withstand higher pressures, among others as described in this invention. If the plug fails, lost circulation and drilling non-productive time results.
The strength of the plug may depend on, inter alia, keeping the induced fracture propped open and/or preserving the increased circumferential (hoop) stress that was required to open the fractures and/or isolating the fracture tips from the fluid and pressure of the wellbore. FIG. 4 provides an illustration of some of the downhole pressures relating to wellbore strengthening. FIG. 4 also illustrates isolation of the fracture tips from the wellbore by plugs comprising wellbore strengthening materials. Understanding how plugs comprising wellbore strengthening materials react to the various pressures experienced in a wellbore may advantageously allow for the design of wellbore strengthening fluids or additives thereof that better strengthen the wellbore, thereby minimizing fluid loss and consequently reducing rig downtime and costs.